This invention relates to processes and plants for drying solid wood in planks or semifinished products by means of a superheated steam system.
In known processes of this kind, which in fact have been practically superseded, the batch of wood to be dried is placed in a cell, and drying is carried out by adjusting only the steam temperature. This drying temperature is chosen according to the species and thickness of the wood. In the case of planks of the resinous species (for example, pine) up to a thickness of 40 mm, the temperature can reach a maximum of 120.degree. C., whereas in the case of thicker planks it must not exceed 110.degree. C.
Dense hardwood having a humidity of 40-60% and tending to collapse has to remain just a few degrees above 100.degree. C., with the temperature being increased towards the end of drying.
In all cases care must be taken that air does not enter the drying cell, because if air enters even in a small percentage (for example, 10%) at a temperature close to 100.degree. C., the hygroscopic equilibrium humidity of the wood falls to such a low value as to immediately damage the wood.
In this respect, the air contained in the cell must be evacuated as far as possible at the beginning of drying, as the entire process has to take place in the absence of air so as not to seriously compromise the result of the drying.
In a pure steam atmosphere, the hygroscopic equilibrium of the wood depends exclusively on the steam temperature. The equilibrium humidity is already 14% at 102.degree. C., whereas at 105.degree. C. it is 10% and at 120.degree. C. is 14%. Consequently, in known processes, in order to prevent undesirably high humidity gradients, the steam temperature has to be adjusted to a value just a little above 100.degree. C. while the wood is still humid, the temperature being increased only towards the end of drying.
In known processes, certain specific characteristics apply. In a first stage, the surface of the wood reaches 100.degree. C. because as the surface water evaporates it prevents the surface temperature from increasing. This phenomenon lasts while the water from the inside percolates to the surface, and this happens for some time because the movement of water from the inside towards the outside is very active because of the high temperature.
Immediately afterwards, when the average humidity of the wood reaches around 40%, a second stage begins in which the evaporation moves deeper. The temperature at the surface, which is now dry, begins to rise beyond 100.degree. C. whereas the temperature in the interior remains close to 100.degree. C.
In a subsequent third stage, the water boils throughout the whole mass of wood, and the temperature in the most inner layers begins to rise beyond 100.degree. C.
In these known processes, one of the most dangerous operations is the preheating, because the internal temperature of the wood is much below its surface temperature. For this reason, it is necessary to prevent surface drying until a pure steam atmosphere is attained and a temperature of 100.degree. C. is reached in the centre of the wood.
Moreover, the second and third of said stages place the wood under critical conditions, as the surface falls to low humidity values even if its temperature rises only slightly above 100.degree. C. (e.g. 5% at 115.degree. C., this representing an advanced shrinkage condition), whereas the most inner layers are generally above the saturation point (zero shrinkage condition).
It is natural that under these conditions, the surface layers of the wood are in a state of high tension, and consequently the inner layers are in a state of high compression. All this happens over a temperature range in which the plasticisation of the wood is reduced (as will be described hereinafter), because of which it is not possible to prevent internal tension and splitting of the wood.
For these reasons, the upper temperature limit of 120.degree. C. is considered impassable in the case of known processes.
With regard to the structure of dryers for carrying out known processes, a brick construction has been superseded because it easily perishes and because the dryer structure has to be absolutely hermetic so as not to allow air to enter. Because of this a metal insulated structure has been adopted in an attempt to completely prevent any steam condensation on the cell walls.
However with this structure it has not been possible to prevent steam condensation on the walls, and it has been difficult to eliminate thermal gradients towards the outside, this being a further cause of condensation and thus of heat dispersion and corrosion. The dryer interior has had to be constructed of aluminium at least 99.8% pure or of stainless steel, because of which the dryer cost is very high.